This invention relates to molybdenum compounds and their methods of making. More particularly, this invention relates to heteropoly molybdenum compounds having a Keggin-type structure.
Ammonium dimolybdate (ADM), (NH4)2Mo2O7, and ammonium paramolybdate (APM), (NH,)6Mo7O24.4H2O, are well-known molybdenum compounds which are used as raw materials in the manufacture of molybdenum metal and molybdenum carbide powders. See, e.g., U.S. Pat. No. 4,079,116. Both compounds are highly soluble in water, up to 200 g Mo/l, and are formed primarily by crystallization from aqueous solutions of ammonium molybdate. The efficiency of the crystallization process and the achievable purity of these compounds is affected by their high aqueous solubility. Because the solutions must be highly concentrated to induce crystallization, the supersaturation point for some of the impurities is also reached resulting in co-crystallization. In addition, common ion effects further induce co-crystallization of impurities, particularly anionic species. Therefore, to avoid incorporation of impurities in the crystallized product, only 70 to 80% of the molybdenum values are typically crystallized from the solutions.
Both of the ADM and APM compounds also contain relatively high percentages of ammonia which can be problematic when directly reducing the compounds to form molybdenum metal. In particular, the large amount of ammonia released during reduction is an environmental concern requiring its removal from the exhaust stream and disposal of the associated waste products.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a molybdenum compound for making molybdenum metal and carbide powders.
It is yet another object of the invention to provide a molybdenum compound with can be manufactured efficiently and at high purity.
In accordance with one object of the invention, there is provided a molybdenum compound having the general formula (NH4)2Mo12MoO40.6H2O wherein the compound is a dodecaheteropoly acid salt having a Keggin-type structure with molybdenum at the hetero and peripheral atomic positions.
In accordance with another object of the invention, there is provided a method for forming the molybdenum compound wherein an ammonium molybdate solution is acidified to obtain a normality of from 0.5 to 1.5 N, and the molybdenum compound is crystallized therefrom.